One embodiment relates to a reception system for frequency-modulated or phase-modulated high-frequency signals for vehicles having a multi-antenna system 1 having at least two antennas and having at least one switch or switching element and at least one individually adjustable phase rotation element, of a linear combination circuit situated in the reception path of at least one of the at least two antennas.
An example of a known antenna diversity system is found from the disclosure of U.S. Pat. No. 5,517,686, U.S. Pat. No. 4,079,318, as well as DE 43 26 843. These diversity systems are aimed at achieving a greater combined signal, by means of equiphase superimposition of two or even more reception signals, than with an individual antenna, in order to thereby reduce the likelihood of level collapses in a territory with multi-path propagation. In this way, a more advantageous signal/noise ratio is obtained in the combined signal, with regard to the receiver noise, on average.
U.S. Pat. No. 5,517,686, provides an example of equiphasing of two reception signals which is produced by means of using an ancillary modulation. For this purpose, one of the two reception signals is modulated in amplitude. In the reception path of one of the two antennas Ant1, Ant2 in U.S. Pat. No. 5,517,686 there is a controllable phase rotation element. If the two reception signals are superimposed in the receiver, the frequency modulation in the sum signal produced by means of the ancillary modulation only disappears if the two reception signals possess the same phase. If the two phases of the superimposed reception signals are not the same, then the sum signal is additionally modulated in frequency with the tone of the ancillary modulation. The tone of the ancillary modulation is detected in the frequency demodulator using a regulation device, and the phase rotation element is adjusted, by means of the regulations, in such a manner that the tone of the ancillary modulation in the sum signal disappears, and thus the two reception signals are superimposed with the same phase. This method according to the state of the art possesses the significant disadvantage, however, that an additional signal in the form of an ancillary modulation has to be imposed onto the reception signals to be received, in order to bring about equiphasing. In DE 10 2006 057 520, a reception system with equiphasing of reception signals E1, E2 . . . EN is described, which advantageously makes do without such ancillary modulation. A reception system of this type can particularly be used in mobile systems with digitally modulated signals, according to the MPSK method, such as, for example, in modern radio satellite reception systems according to the QPSK method. Equiphasing is achieved by means of successive separate determination of the voltages of the different antennas Ant1, Ant2 . . . AN, using an algorithm for calculating the phases that are turned on in the different antenna branches. A diversity array of this type is connected with the problem that weak reception signals E1, E2 . . . EN having a large noise component do not precisely allow equiphasing, and that the signal-to-noise ratio (SNR) in total is too poor. This particularly holds true in reception territories with marked multi-path propagation with strong reception level collapses. In DE 10 2006 057 520, several methods for equiphasing are described, which is brought about using the power levels of the reception signals of the antennas Ant1, Ant2 . . . AN.
It is known that the data stream of every digital signal transmission contains signals referred to as “burst signals” or “frame data” for synchronization of the transmission, which are established in accordance with the appropriate standard, and repeatedly transmitted at the time interval of the frame period TR. The burst signal contains symbols that contain the reference phase 80 for phase synchronization of the system. In particular, in order to assure reliable synchronization to the symbol cycle even at high speeds, both the frame frequency and the burst signal period TB of the burst signal, in terms of time, must be selected to be appropriately large. The data contained in the symbols received between the burst signals thus consists of the current phase deviation, in each instance, of the high-frequency carrier oscillation from the reference phase 80 present in the receiver, which is derived from the burst signals. The receiver-side detection of the carrier phases contained in the useful symbols transmitted can then take place in the receiver, in secured manner, if the signal/noise ratio (SNR) is sufficiently great and the system is synchronized to the symbol cycle by means of the burst signals. A fundamental problem, however, results from the fact that in the case of mobile reception, the carrier phase varies greatly, over the travel routes, in a reception field in which interference results from multi-path propagation, whereby these variations lead to incorrect detection of the symbols, for example if, in the case of a 4PSK system, for example, the error phase deviation is not recognizably less than ±π/4. The reference phase of the reception signal is present in the receiver in constantly updated form, on the basis of the burst signals that are received at sufficiently short time intervals. This means that the phase of the transmission function of the radio field and thus of the reception signal, which constantly changes in mobile operation, is always correctively rotated in the receiver, using the reference phase 80, in such a manner that the individual symbols that carry the data are assigned in each instance to the correct phase quadrant with regard to this reference phase 80, if this signal is present at a sufficiently great SNR. The combined signal 8 of the multi-antenna system 1 is maximal when the individual reception signals E1, E2, . . . EN in the combination circuit are superimposed with the same phase, in a common phase location—referred to as the nominal phase—the difference of which, in the receiver, at any point in time, relative to the reference phase 80 present there, represents the symbol data. The individual reception signals E1, E2 . . . EN are therefore supposed to be superimposed under a common nominal phase, in the combined signal 8. The nominal phase is therefore a phase that is common to all the reception signals E1, E2 . . . EN, by means of a corresponding setting of the phase rotation elements, which phase possesses a value that is at first indefinite with regard to the reference phase. At the moment of reception of reference symbols in a burst signal, their system-specific phases thus yield the reference phase 80 in the receiver, up to the next update of the reference phase 80 by means of subsequent burst signals with additional reference symbols. Particularly, in the case of reception in poorly supplied territories, the reception signals E1, E2 . . . EN are frequently very noisy, so that the reception mechanism described is not error-free and the system, because of frequent incorrect detection of the symbols overly great bit error rates occur, which desynchronize the system.